1. Field of the Invention
The present invention relates to a frequency-tunable filter, and more particularly to digitally-controlled, small-sized, low power-consuming tunable wideband filter and tunable multi-band wideband filter using switched capacitors and inductors in use of the same, which are suitable for wideband communication systems and multi-mode communication systems.
2. Description of the Related Art
In general, frequency-tunable bandpass filters with freely variable center frequencies are required in wideband communication systems, such as television (“TV”), ultra highband (“UWB”), and multi-mode communication systems, such as cellular phones, personal communication service (“PCS”), and wideband code-division multiple access (“WCDMA”).
FIG. 1 is a block diagram showing a communication system of a conventional RF receiver. In FIG. 1, an RF band signal received through an antenna is input to an RF bandpass filter 10. The RF bandpass filter 10 filters the signal of the antenna to a wireless frequency band, and outputs the filtered signal to a low-noise amplifier (LNA) 20. The low-noise amplifier 20 receives a signal output from the RF bandpass filter 10, low-noise-amplifies the signal by a predetermined gain, and outputs the amplified signal to a mixer 30. The mixer 30 mixes the signal of the low-noise amplifier 20 with a sinusoidal signal generated from a local oscillator, and outputs a signal having a center frequency fIF shifted in a center frequency band. The signal of the mixer 30 is input to the base band modem 60 through an intermediate frequency filter 40 and an intermediate frequency/automatic gain control amplifier 50.
As such, the conventional RF receiver has the RF bandpass filter 10 with fixed frequency characteristics.
FIG. 2 illustrates a general multi-band and multi-mode receiver that switches receivers fit to a band and mode. Thus, the multi-band and multi-mode receiver requires additional receivers and space.
The development of integration circuit (“IC”) technology is facilitating the implementation of wideband low noise amplifiers (“LNAs”) and local oscillators. As shown in FIG. 3, a frequency-tunable radio frequency (“RF”) bandpass filter enables a single receiver to serve as a multi-band and multi-mode receiver.
The frequency-tunable RF bandpass filter enables a user to select a desired RF band channel. The frequency-tunable RF bandpass filter removes interference signals outside of the desired band and interference signals caused by neighboring channels which alleviates the linear characteristics required for an RF circuit and the phase-noise requirements of a local oscillator so that power consumption is reduced.
As previously discussed, the conventional frequency-tunable bandpass filter is constructed in the hybrid form using a varactor diode controlled by an analog tuning signal or a pin diode switch. However, the conventional frequency-tunable bandpass filter described above has severe non-linear characteristics, a large size, increased power consumption, and is costly.
On the other hand, a frequency-tunable RF bandpass filter using only active elements is easily controlled; however, it consumes a lot of power, has a low yield, and performs worse than the passive filter in terms of linearity or noise factor. As such, the frequency-tunable bandpass filter using only active elements is difficult to apply to the wideband communication systems or multi-mode communication systems.